1. Field of the Invention:
This invention relates to a method for dispersing suspensions and an apparatus for practicing this method.
More particularly, the invention relates to a novel method for dispersing suspensions, which comprises charging a suspension of solid particles such as paints and inks in a cylindrical vessel packed with a number of small grinding media, mixing the suspension with the grinding media, and agitating the suspension together with the grinding media by means of an agitator having one agitation rod, which is disposed in the interior of said vessel, to thereby disperse the suspension in the colloidal state, and to an apparatus for use in practicing this method.
2. Description of the Prior Art:
As dispersion means for dispersing suspensions such as paints and inks, there have heretofore been employed an apparatus having an agitation shaft provided with one or several discs or vanes having a specific shape and an apparatus having an agitation shaft provided with several agitation rods. As typical instances of such conventional apparatus, there can be mentioned a sand mill (disclosed in the specifications of U.S. Pat. Nos. 2,581,414 and 3,185,398), an Attritor (trademark for a dispersion apparatus manufactured by Union Process Company, U.S.A.), and the like.
In the sand mill, a suspension is continuously fed into the interior of a cylindrical vessel from the bottom thereof, and during passages through the interior of the vessel, the suspension undergoes the action of a number of grinding media (for example, glass beads or synthetic ceramic balls having an average size of 0.7 to 3.0 mm) packed in the vessel and several disc-like agitation vanes provided on an agitation shaft disposed in the cylindrical vessel and rotated at a high speed corresponding to a peripheral speed of at least about 10 m/sec, and suspension is dispersed by high "shearing stress" developed by the grinding media. The dispersed suspension is separated from the grinding media by means of a mesh screen mounted in the interior of the cylindrical vessel, and then it is withdrawn in the dispersed state.
The dispersion apparatus of this type, however, includes defects such as mentioned below:
1. Unless the agitation shaft is rapidly rotated at a velocity of at least 800 rpm and a peripheral velocity of at least about 10 m/sec, the suspension is not effectively dispersed. This structural limitation results in the following disadvantages:
1. Because of excessive wearing of the inner wall of the cylindrical vessel, the mesh screen and the grinding media, the maintenance cost becomes high.
2. In the case of a suspension of a high content of solid particles or a suspension of a high viscous resistance, the temperature is extremely elevated, and in some cases, the degree of the temperature elevation is extremely high. Therefore, sufficient temperature control cannot be accomplished, with the result that a great deviation is brought about in the quality of products. Accordingly, this apparatus cannot be applied to suspensions including a composition having a high temperature sensitivity.
3. When an agitation shaft provided with several disc-like agitation vanes is rotated at a high speed, a high agitation resistance is imposed and much power is required for the high speed agitation. Accordingly, the capacity is naturally limited in the dispersion apparatus of this type, and the maximum capacity is about 250 liters in commercially available apparatuses of this type.
2. As is apparent from the dispersing mechanism in the apparatus of this type, it is difficult to disperse a suspension containing solid masses and large agglomerates, and therefore, it is sometimes necessary to disentangle such masses or agglomerates sufficiently in a preliminary kneading step prior to the treatment by the apparatus.
3. Some solid materials cannot be finely dispersed by the apparatus of this type, and in such case it is necessary to use another dispersing machine in addition to the apparatus of this type.
4. The range of a viscosity of the suspension applicable to the apparatus of this type is very narrow, and unless the viscosity is within a range of 85 KU to 120 KU (as measured at 25.degree.C. by a Stormer viscometer), sufficient dispersion is not obtained or it takes a long time to complete dispersion.
5. Unless the specific gravity of the suspension is lower than that of the grinding media contained in the cylindrical vessel, it is quite impossible to accomplish dispersion or it takes a long time to complete dispersion.
6. Provision of accessory equipment such as the above-mentioned pre-treatment equipment (for preliminary kneading) and the post-treatment equipment (for dissolution and color adjustment) is indispensable in the dispersion apparatus of this type. Therefore, the equipment cost becomes high and a large space is necessary for accommodation of the apparatus of this type.
In the Attritor, several columnar agitation rods are provided at prescribed intervals with a certain angle on an agitation shaft disposed in a cylindrical vessel, and the agitation shaft is rotated at a relatively low speed corresponding to a peripheral speed of about 3 m/sec. By the rotation of this agitation shaft, as in the case of the above-mentioned sand mill, a number of grinding media (for example, synthetic ceramic or steel balls having an average size of 3 to 12 mm) and the suspension undergo the moving action of the agitation shaft, and the suspension is dispersed by high shearing stresses formed by the grinding media.
The dispersion apparatus of this type, however, includes the following defects:
1. Although agitation is conducted at a relatively low speed, since relatively heavy media such as synthetic ceramic or steel balls having an average size of at least 3 mm are employed, much power is required for agitation and hence, the capacity of the apparatus is naturally limited. The maximum capacity is about 1000 liters (net capacity of 600 liters) in commercially available apparatuses of this type.
2. Since grinding media composed of a heavy and rigid material are employed, the inner wall of the vessel is readily worn and such disadvantages as contamination and discoloration of products are brought about by incorporation of iron powder.
3. Prior to the treatment by the apparatus, it is necessary to disentangle masses and agglomerates and make the suspension homogeneous in a pre-treatment step (preliminary kneading).
4. Convection currents of the suspension are so mild that the apparatus cannot be used as a mere mixer (for incorporation of a pigment or the like).
As another instance of a dispersion apparatus recently developed, there can be mentioned the dispersion apparatus marketed under the tradename of "Polymill" by J.H. Day Company (disclosed in Paint and Varnish Production, October issue, 1972, pages 6 to 7, published by Palmerton Publishing Co., Incorp., U.S.A.). Glass bead grinding media are employed also in this apparatus, and the suspension is dispersed by rotating a disc having a specific convex and concave shape in a mixture of the grinding media and suspension. Defects involved in the above-mentioned sand mill are similarly observed in the dispersion apparatus of this type.
Further, there is known an apparatus in which the suspension is dispersed by agitation of one disc impeller (see U.S. Pat. No. 3,416,740), a dispersion method in which the suspension is agitated in grinding media with several agitation rods mounted on one agitation shaft in random directions (see U.S. Pat. No. 2,764,359), and similar apparatuses and methods. However, the above-mentioned defects cannot be overcome by any of these conventional techniques.